It can kill birds at 60 yards.... but I won't. Simp has a real nailer of a turkey gun as well. My shells are expensive and his are worse! I suspect if you underwrite his ammo cost, he'll show you some impressive stuff from 60 yards.
My gun is a Mossberg835 Grand Slam. I shoot a Pure Gold .670 choke tube. My shell of choice, after patterning damn near everything made except the new heavier payload hevi-13 and Nitros (which I suspect would out do what I have now)... is Winchester Elite Extended Range 3.5" 2oz of #6 shot.
There are a couple of choke tubes to consider. Pure Gold is definately one, not just because thats what I have, but because they give you a money back satisfaction guarentee. So as long as you pattern within 30 days of purchase, if the Pure Gold isn't the top performer, you'll get your money back. Thats a can't lose. Comp-n-choke, Rhino and as of late I've been hearing good things about Indian Creek, are also reputable, super tight, long range shooters. My best friend has an 835 also that will put mine to shame. He has a choke tube that was made by a shadetree gunsmith in Carolina County Virginia. He is not a professional in business, so I will not name him publicly without first asking his permission. He took Kevin's gun and just tinkered with different choke tubes (he makes them himself) until he found the one that was just right. I don't know what the constriction is, what its made of or anything. Its flush fit, plain silver, looks like a $5 used bin choke tube. Kevin gave $40 dollars for it and I've never in person seen its equal out of a 12 gauge. He has been/was shooting Remington Hevi shot 3.5" #6s. If the rain holds out, we are going to shoot some Hevi-13 tomorrow, in addition to raiding a seldom if ever fished pond full of slab sided crappie.
My patterns are dense enough to knock'em down at 60. Will #6 Extended Range carry enough density.... maybe.... personally I'll never find out for sure. Looking at figures it should have the energy to get it done. If you are looking for a pellet that will get the job done, if you aren't shooting a high density shot, then you had better be shooting lead #4s. The only issue with #4s is your pellet count drops substantially, and so will your pattern density. The only two patterns I have out of that gun at 60 yards (which was really just to see what the gun was made of) gave me something like 93 and 112 hits in a 10" circle. They are buried upstairs someplace.
The best advice I can give you from personal experience, if you want a shotgun that will kill a turkey at 60 yards:
buy a 10 gauge. My gun shoots extremely well for a 12 gauge, and I'd say that Kevin's gun is probably 1 in 1000, but compared to a 10, I might as well be shooting a 20 gauge and his would be mediocre at best. The Nitro shells that Simp and a few others here on the board shoot are even more expensive than Win. Ext Range (which itself is around $3.50-$3.65 per shell). But I will testify that EVERY SINGLE PERSON I KNOW OR HAVE SPOKEN WITH THAT HAS EVER SHOT NITRO SHELLS..... IS STILL SHOOTING THEM. And that is an endorsement that you can't make up or buy with anything except consistant top performance.
You could always shoot a .22-250 as well.